Helium

On 18 August 1868 a total solar eclipse crossed India, and Jules Janssen, observing from Guntur, turned a spectroscope on the prominences at the Sun's edge. Among the expected lines he recorded a bright yellow one at a wavelength close to, but not the same as, the familiar sodium pair. In October of the same year Norman Lockyer in London, using a method that let him observe prominences without an eclipse, saw the same line. Lockyer and Edward Frankland concluded that no known element produced it and proposed a new one, named for Helios.

The claim was not well received. Announcing an element observed only in the atmosphere of the Sun, with no sample, no weight and no chemistry, was a considerable liberty, and for a quarter of a century helium was a line in a spectrum and an argument about it.

The matter was settled in 1895. William Ramsay, treating the uranium mineral cleveite with acid, collected a gas showing exactly Lockyer's yellow line; Per Teodor Cleve and Nils Abraham Langlet in Uppsala reached the same result independently and almost simultaneously. Helium was real, it was on Earth, and it had been sitting inside uranium ore the whole time.

That last detail is the explanation. An alpha particle is a helium nucleus. Every alpha decay in the crust makes another helium atom, and the gas accumulates in the same porous rock traps that hold natural gas. Terrestrial helium is therefore not primordial but a by-product of radioactive decay, manufactured slowly over billions of years and, once released into the air, lost to space permanently. The world's supply comes from a small number of gas fields, and it is finite in a way that most resources are not.

Helium is inert in a stronger sense than the other noble gases: no stable compound of it is known under ordinary conditions. Its two electrons fill the first shell completely, and there is nothing whatever for chemistry to take hold of.

Physically it is stranger. Helium boils at minus 268.9 degrees C, about 4.2 degrees above absolute zero, the lowest boiling point of any substance. Below about 2.17 degrees above absolute zero, liquid helium-4 becomes a superfluid: it flows without viscosity, climbs the walls of its container as a thin film and escapes over the rim, and conducts heat so efficiently that it stops boiling and goes glassily still. Pyotr Kapitsa and, independently, John Allen and Don Misener reported the effect in 1938.

Alone among the elements, helium does not freeze at ordinary pressure however cold it is made. Its atoms are so light and their mutual attraction so weak that quantum zero-point motion keeps them mobile down to absolute zero; roughly twenty-five atmospheres are needed to force a solid. This is not a curiosity at the margin of physics but the clearest visible case of quantum mechanics governing a bulk quantity of matter that could be poured from a jug.

The practical consequences are large. Liquid helium cools the superconducting magnets of MRI scanners and particle accelerators, and there is no substitute for it. Breathing a helium and oxygen mixture avoids nitrogen narcosis in deep diving. The high voice comes from sound travelling nearly three times faster in helium than in air, which shifts the resonances of the vocal tract without altering the pitch at which the vocal cords vibrate.

Nothing traditional attaches to helium, and nothing could: it was unknown until 1868 and had never been held in the hand until 1895.